The solar cycle is a 28-year cycle of the Julian calendar with respect to the week. It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28 year sequence. Calendar years are usually marked by Dominical letters indicating the first Sunday in a new year, thus the term solar cycle can also refer to a repeating sequence of Dominical letters. Unless a year is not a leap year due to Gregorian exceptions, a sequence of calendars is reused every 28 years. The name solar cycle comes from Sunday, the traditional first day of the week.